I heard The Conduit does a solid job on the Wii as a FPS.
Also, if you plan to grab No More Heroes like some people have recommended, grab the Red Steel/NMH double pack. I actually enjoyed Red Steel very much.
And yeah, every first party Nintendo game out there warrants a purchase. Smash Bros., Mario Galaxy, New Mario Bros, Mario Kart, Metroid Prime Trilogy, Twilight Princess, etc.
Except Animal Crossing. Least improved game of the year.
I'm a bit of an eclectic gamer, so I can't really throw down some good (mainsteam) suggestions, although...
If you're into bullet hell type shooters, Castle of Shikigami 3 will totally satisfy any danmaku enthusiast.
If you have any friends, Dokapon-freaking-Kingdom. The game is great, but only if you have friends to play it with. The AI cheats. Literally. Or so I heard from some of my friends.